Originally posted by pauluk:
A push-pull audio output stage can operate in class A, B, or A/B.
Class A means that both transistors (or tubes) are conducting all the time. As one conducts more heavily the other conducts less, and vice versa.
With class B, the transistors are biased to cut-off point, so with no signal there is no (or very little) current. One transistor then conducts for positive parts of the signal, the other for negative portions.
Class A generally offers better fidelity, but because of the quiescent current it is much less efficient than class B.
Class A/B is a compromise between the two. With a low-level signal, both transistors conduct and the amplifier works in class A. At higher levels, one transistor cuts off during positive peaks, the other during negative, effectively changing the mode to class B.
While class A/B isn't as efficient as class B, it avoids crossover distortion which can sometimes be a problem with class B.
Thanks for the much more informed responseto the original post, Paul. Am I being misleading about one class of amp being transformerless, or is it a different animal altogether?
